Chemical Vapor Deposition
Lecture date: Monday, 2014.09.29 (lecture recording)
CVD is used to grow thin films using chemical reactions involving gaseous precursors.
CVD takes place in a flow reactor; the product is the thin film.
The precursors may be activated in several different ways:
- heating of the substrate ("cold wall")
- heating of the entire reactor ("hot wall")
- heating of the gas ("hot filament")
- with plasma
- ions and electrons aid in low-temperature activation
- with light
- photoactivation with lasers or other light sources
Transport and Reaction Considerations
Reactions in both the gas phase and on the surface
Transport—thermal and mass—in the gas phase (convection and diffusion) and to the surface
A boundary layer will form as the gas flows over the treated surface; as boundary layer thickness increases, the resistance to transport to the surface also increases.
Deposited Film Properties
When depositing a film, you're worried about:
- stress (due to differences in specific volume or crystal lattice structure)
- topography (conformal? hole-filling?)
To deposit SiO₂, there are several options of precursors:
- SiH₄ + CO₂ + H₂
- SiCl₂H₂ + N₂O
- SiH₄ + N₂O
- SiH₄ + NO
- SiH₄ + O₂
While all of these combinations may result in the same film, the film properties will be different!
A common strategy to relax stresses when growing SiO₂ films is to operate at a temperature sufficiently high to keep the SiO₂ viscoelastic so that it can flow.
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